The 3rd Generation Partnership Project (3GPP) standardization body is currently working on the specification of the evolved 3G mobile system, where the core network related evolution of the architecture is often referred to as SAE (System Architecture Evolution) or Evolved Packet Core (EPC), while the Radio Access Network (RAN) evolution is referred to as Long Term Evolution(LTE) or Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The name SAE/LTE or Evolved Packet System (EPS) refers to the overall system. The Release 8 specification of the 3GPP standard, which is to be completed in 2008, will include the specification of the SAE/LTE evolved system. For an overall description of the LTE part of the architecture see 3GPP TS 36.300 “E-UTRA, E-UTRAN Overall Description” and for the SAE part see 3GPP TS 23.401 “General Packet Radio Service (GPRS) Enhancements for E-UTRAN Access.”
The SAE/LTE architecture is also often called as a two-node architecture, as logically there are only two nodes involved—both in the user and control plane paths—between the User Equipment (UE) and the core network. These two nodes are the base station, called eNodeB in 3GPP terminology and the Serving Gateway (S-GW) in the user plane, and the Mobility Management Entity (MME) in the control plane. There may be multiple S-GW and MME nodes in a network.
The S-GW executes generic packet processing functions similar to router functions, including packet filtering and classification. The MME terminates the so called Non-Access Stratum (NAS) signaling protocols with the UE and maintains the UE context including the established bearers, the security context as well as the location of the UE.
In the LTE architecture, the radio link specific protocols, including Radio Link Control (RLC) and Medium Access Control (MAC) protocols are terminated in the eNodeB. In the control plane, the eNodeB uses the Radio Resource Control (RRC) protocol to execute the longer time scale radio resource control toward the UE, such as, for example, the establishment of radio bearers with certain Quality of Service (QoS) characteristics, the control of UE measurements or the control of handovers.
The network interface between the eNodeB and the EPC network is called the S1 interface, which has a control plane part (S1-CP) connecting to the MME and a user plane part (S1-UP) connecting to the S-GW. The user plane part of the S1 interface is based on the GPRS Tunneling Protocol (GTP). The tunneling mechanism is needed in order to ensure that the Internet Protocol (IP) packets destined to the UE can be delivered to the correct eNodeB where the UE is currently located. For example, the original IP packet is encapsulated into an outer IP packet that is addressed to the proper eNodeB.
The S1 control plane protocol is called S1-AP and it is carried on top of Stream Control Transmission protocol (SCTP)/IP. The MME uses the S1-AP protocol to talk to the eNodeB, e.g. to request the establishment of radio bearers to support the QoS services for the UE. There is a network interface also between neighbor eNodeBs, which is called the X2 interface, and it has a similar protocol structure as the S1 interface with the exception that the control protocol is called X2-AP. The X2 interface is primarily used for the execution of the handover of a UE from one eNodeB to the other but it is also used for the inter-cell coordination of other Radio Resource Management functions, such as Inter-Cell Interference Coordination. During a handover execution, the source eNodeB talks to the target eNodeB via the X2-AP protocol to prepare the handover and during the handover execution it forwards the pending user plane packets to the target eNodeB, which are to be delivered to the UE once it has arrived at the target eNodeB. The packet forwarding is done via the X2 user plane which is using the GTP tunneling protocol similar to the user plane on the S1 interface.
The network infrastructure that is used to connect the different network nodes, e.g., the eNodeB's, MME's and S-GW's, is an IP based transport network, which can include L2 networks with different technologies, i.e., SDH links, Ethernet links, Digital Subscriber Line (DSL) links or Microwave links, etc. The type of transport network and L2 technologies employed is a deployment issue, depending on the availability, cost, ownership, operator preferences, etc., of such networks in the particular deployment scenario. However, it is generally true that the costs related to the transport network often play a significant part of the overall operation costs of the network.
In a further enhancement of the LTE system, called LTE-Advanced. 3GPP discusses possible solutions to use the LTE radio interface from an eNodeB not only for serving UEs but also for serving as a backhaul link to connect to other eNodeBs. That is, an eNodeB can provide the transport network connectivity for other eNodeBs utilizing an LTE radio connection via the other eNodeBs. This method is called self-backhauling since the radio link itself is used also as a transport link for some of the base stations. In an LTE system employing self-backhauling, an eNodeB that is connected to the network via a radio connection is referred to as self-backhauled eNodeB, or B-eNodeB for short, while the eNodeB that is providing the backhaul radio connection for other eNodeB(s) is called the anchor eNodeB, or A-eNodeB for short (“eNodeB,” by itself, refers to regular eNodeBs, which are neither self-backhauled nor anchor eNodeBs).